The inner walls of the cylinder bores of internal combustion engines are required to withstand the abrasive action of the piston and its seal rings. In models with cast iron engine blocks, the cast iron provides the required resistance. In other models, including some V-engine blocks in which aluminum or other lightweight material is used, cylinder liners are inserted in the bores to provide adequate wear resistance.
In many engine block casting processes, cylinder liners are an integral part of the casting process and are assembled into the mold before molten metal is introduced into the mold cavity to form the engine block. After casting, when the mold is removed, these cast-in liners are permanently embedded within the cast metal walls of the cylinder bores. To improve the mechanical contact between the cylinder liners and the walls of the cylinder bores and avoid imperfections that are caused by thermal variations between the cylinder liners and the molten metal, the cylinder liners are sometimes pre-heated using, for example, induction heaters.
In a sand casting process, often referred to as the Precision Sand Process, an expendable mold package or package subassembly 40, shown in FIG. 1, is assembled from various mold segments and mold cores 44 that are combined to define, together with the cast-in cylinder liners 46, the internal and external surfaces of the engine block. The mold segments and mold cores are made of resin-bonded sand. Proper positioning of the liners in the mold and prevention of migration of the liners during pre-heating and casting presents an ongoing challenge.
Some attempts to address this problem provide that chamfered cylinder liners remain seated on corresponding chamfered seat surfaces of the mold cores during thermal expansion. The prior art provides for chamfered surfaces that are inclined with respect to a plane perpendicular to the bore axis at specific angles that are calculated to ensure that the liners remain seated and in contact with seat surfaces during pre-heating and casting. These angles are calculated using nominal (theoretical) dimensions for the length and radius of the cylinder liners and assume uniform in-situ thermal expansion of the liners. In practice, these ideal conditions are not met and the variation can cause the cylinder liners to exert force against the constraining mold seats. As a result, the mold seats will move relative to one another and/or the resin-bonded sand will fracture or crush, contaminating the mold. Either of these unintended consequences is undesirable and potentially more catastrophic than a small amount of cylinder liner migration.
Therefore, improved casting molds with cast-in cylinder liners are still needed.